The invention relates to a method of detecting a watermark in an information signal that has possibly been watermarked by modifying values of said information signal in accordance with (temporally or spatially) corresponding values of a watermark pattern. The invention also relates to an arrangement for detecting a watermark.
A prior art method as defined in the opening paragraph is disclosed in International Patent Application WO-A-98/03014. The watermark is detected by computing the correlation of the suspect information signal with an applied watermark pattern, and comparing the correlation with a predetermined threshold. If the correlation is larger than the (threshold, the watermark is said to be present, otherwise it is said to be absent. The larger the correlation is, the more reliable the detection is and the more processing is allowed until the watermark can not be detected anymore. As disclosed in WO-A-98/03014, the information signal and/or the watermark pattern may be subjected to matched filtering before the correlation is computed. The reliability of the watermark detection is drastically improved thereby.
A problem of the prior art watermark detection method is that the (temporal or spatial) position of the watermark pattern with respect to the information signal is not absolutely known. If the position of the watermark applied to the detector differs from the location during embedding, the correlation will be small and the watermark can erroneously detected to be absent.
It is an object of the invention to further improve the method of detecting a watermark.
This is achieved by the method defined in claim 1 and the arrangement defined in claim 6. Advantageous embodiments are defined in the subclaims.
The invention exploits the insight that the correlation of the information signal and the applied watermark for a number of possible positions of the watermark is best computed in the Fourier domain, and that the robustness and reliability of detection can be improved by applying Symmetrical Phase Only Matched Filtering (SPOMF) to the information signal and the watermark before correlation. SPOMF, which is known per se in the field of pattern recognition, postulates that most of the relevant information needed for correlation detection is carried by the phase of Fourier coefficients. In accordance herewith, the magnitudes of the complex Fourier coefficients are normalized to have substantially the same magnitudes.
Although the adjective xe2x80x9csymmetricalxe2x80x9d in the expression SPOMF refers to filtering both the information signal and the watermark, the inventors have found that normalizing the Fourier transformed information signal contributes most to the improvement of the correlation detection. Also normalizing the magnitudes of the Fourier transformed watermark constitutes an embodiment of the invention.
The step of normalizing the magnitude of complex Fourier coefficients includes dividing each Fourier coefficient by its absolute value. However, the inventors have found that the detection strength does not decrease significantly if each coefficient is divided by its real or imaginary part, depending on which is the largest. This yields a courser approximation for the normalization (the magnitudes will vary between 1 and {square root over (2)}), but reduces the number of calculations considerably.